1. Field of the Invention
The present invention relates to a technology for reducing the vibrations of an electrically driven actuator.
2. Description of the Related Art
For an actuator which transmits the output of a motor through a gear device to a threaded shaft and which sets up a linear motion in a member engaged with the threaded shaft, it is preferable, in view of facilitating miniaturization, to use a gear device in which an input and an output shaft have coincident axes. As a gear device of this type, a planetary gear mechanism is widely known in the prior art. The planetary gear mechanism in general consists of three elements, i.e., a sun gear, a planetary gear and a carrier. The sun gear includes an external gear and/or an internal gear (a ring gear). The planetary gear mechanism is classified into various types according to the combination of these three elements.
FIG. 7 shows a gear device called an S-P-C type planetary gear mechanism. In FIG. 7, reference symbol S designates a shaft of a sun gear, P designates a shaft of a planetary gear, and C designates a shaft of a carrier. These shafts are arranged as a basic structure. In an example shown in FIG. 7, the shaft C of the carrier serves as an input shaft, and the shaft P of the planetary gear serves as an output shaft. The shaft C of the carrier eccentrically supports the planetary gear and makes it rotatable. The planetary gear makes rotating and revolving motions by engaging with the fixed sun gear (the ring gear). The shaft P of the planetary gear is provided with a universal joint J or a linking element equivalent thereto so as to pick up only the revolving motion as an output.
The S-P-C type planetary gear mechanism is highly useful in various fields since a relatively large speed reduction ratio (30:1) is attainable in said mechanism by means of only two gears (the ring gear and the planetary gear) in a state wherein the input shaft is arranged coaxially to the output shaft. In addition, a so-called harmonic drive is a gear device classified as an S-P-C type planetary gear mechanism.
For the S-P-C planetary gear mechanism of this type, it is essential to suppress the vibrations generated by an eccentric motion of the planetary gear. FIG. 8 shows a conventional S-P-C type gear device constructed for the purpose of suppressing the generation of the vibrations.
A gear device 1 is designed such that a fixed ring gear 4 and planetary gears 5 and 5xe2x80x2 which make the rotating and revolving motions are disposed between an input shaft 2 and an output shaft 3, to thereby perform a speed reduction of one step. The input shaft 2 is provided with eccentric portions 2a and 2axe2x80x2, where the planetary gears 5 and 5xe2x80x2 are supported on the shaft 2 eccentrically. As shown in FIG. 8(a), if the eccentric portion 2a is arranged eccentric with the axis of the input shaft 2 by a distance a, correspondingly the eccentric portion 2axe2x80x2 is also arranged to be eccentric therewith by the distance a. Hereby, the planetary gears 5 and 5xe2x80x2 counteract each other""s vibration when they make the rotating and revolving motions in engagement with the ring gear 4.
As a linking element corresponding to the universal joint J shown in FIG. 7, a coupling plate 7 is disposed between a flange 6 fixed to the output shaft 3 and the planetary gear 5, and key slots 7a are provided on both sides of the coupling plate 7 so as to interact perpendicularly. A key 6a provided on the flange 6 and a key 5a provided on the planetary gear 5 are slidably engaged respectively with the key slots 7a on both sides so as to constitute a universal joint. Although not illustrated, the linking element which transmits the rotating motion of the planetary gear 5xe2x80x2 to the output shaft 3 can be constructed by the same mechanism.
In the gear device 1 shown in FIG. 8, when the planetary gear 5 makes the rotating and revolving motions while being engaged with the ring gear 4, the planetary gear 5 and the planetary gear 5xe2x80x2 function as balancers mutually. Thereby, the planetary gear 5 and planetary gear 5xe2x80x2 counteract each other""s vibration.
However, the planetary gears 5 and 5xe2x80x2 are disposed to be offset from each other in the axial direction (i.e., the axial direction of axes of the input and output shafts 2 and 3). Therefore, the gear device shown in FIG. 8 secures a static balance among the rotating elements, but does not secure an axial dynamic balance among the rotating elements. Accordingly, in a case the gear device 1 shown in FIG. 8 is required to be driven at a high speed or to be operated precisely, there has sometimes arisen an adverse effect due to the vibrations caused by the axial dynamic imbalance among the rotating elements.
The present invention was made in order to solve the above-mentioned problem, and an object thereof is to provide an electrically driven actuator equipped with a gear device which can improve an axial dynamic balance in a so-called S-P-C type planetary gear mechanism and which can suppress the generation of the vibrations under various use conditions. Another object of the present invention is to provide a highly precise and low-cost electrically driven actuator by simplifying the structural complication of the gear device and by providing a high-precision gear device at a low cost.
The electrically driven actuator according to a first aspect of the present invention for solving the foregoing issue is characterized by a gear device which performs a speed reduction of one step using two gears disposed between an input shaft and an output shaft coaxially arranged, and which secures an axial dynamic balance in every individual rotating element in the electrically driven actuator which transmits the output of a motor through the gear device to a threaded shaft, and which generates a linear motion in a member engaged with the threaded shaft.
In the present invention, the axial dynamic balance in the gear device as a whole in the electrically driven actuator is secured by keeping an axial dynamic balance in every individual rotating element which composes the gear device.
In the electrically driven actuator according to a second aspect of the present invention, the rotating elements include a disk which has a circular recess arranged to be eccentric with respect to the input shaft and which is fixed to the input shaft, a ring gear which is rotatably supported by the circular recess and whose rotating motion is restricted to an eccentric motion in the circular recess when the disk rotates, and a planetary gear which is fixed coaxially to the output shaft and engaged with the ring gear. The disk is provided with a weight portion for recovering the axial dynamic balance which is lost by arranging the circular recess and the ring gear.
In this arrangement, the weight portion recovers the axial dynamic balance which is lost by arranging the circular recess and the ring gear. Since the planetary gear is fixed coaxially to the output shaft, it does not make the eccentric motion and thus secures the dynamic balance thereof. Accordingly, the axial dynamic balance is secured in every individual rotating element.
In the electrically driven actuator according to a third aspect of the present invention, the weight portion includes thickness-reduced portions which are formed in an unrecessed portion of the disk, and which have the same depth as that of the recess. According to the present invention, since the weight portion is formed in the disk per se, it is unnecessary to add a new element onto the disk for keeping an axial balance of the disk. Secured by setting the depth of the thickness-reduced portions to be identical with that of the recess is the axial dynamic balance which is lost due to the circular recess on the disk.
In the electrically driven actuator according to a fourth aspect of the present invention, the thickness-reduced portions are a plurality of circular holes having the same diameter. By providing a plurality of circular holes having the same diameter as thickness-reduced portions, the manufacturing process for forming the thickness-reduced portions is reduced.
In the electrically driven actuator according to a fifth aspect of the present invention, the thickness-reduced portions include a plurality of circular holes whose diameters are gradually varied. By providing a plurality of circular holes having different diameters as thickness-reduced portions, more ideal balance can be attained.
In the electrically driven actuator according to a sixth aspect of the present invention, the disk is formed axially dividable so as to increase the degree of freedom in selecting materials for respective portions of the disk. Furthermore, the circular recess and the thickness-reduced portions are easily formed equal in depth by providing a half of the dividably formed disk with the circular recess and the thickness-reduced portions before it is combined with the other half.
In the electrically driven actuator according to a seventh aspect of the present invention, rotating elements include a planetary gear which is axis-supported by an eccentric portion provided on the input shaft, and which makes a rotating motion and a revolving motion, and the planetary gear is provided with a cavity portion within which the weight portion is provided to recover a balance of the planetary gear.
In this structure, by providing the cavity portion in the planetary gear and by locating the weight portion within the cavity portion, the balance is recovered within the planetary gear which is axis-supported by the eccentric portion of the input shaft to make the eccentric motion. That is, by securing the axial dynamic balance of the planetary gear per se, the axial dynamic balance is secured for the entire gear device.
In the electrically driven actuator according to an eighth aspect of the present invention, the weight portion is fixed to the eccentric portion of the input shaft so that the eccentric motion of the center of gravity of the weight portion is in the phase opposite to the eccentric motion of the center of gravity of the planetary gear. According to this structure, the center of gravity of the planetary gear and that of the weight portion are constantly in the position wherein they have an even balance with each other, and therefore it is possible to secure the axial dynamic balance of the planetary gear which is the rotating element of the gear device.
The electrically driven actuator according to a ninth aspect of the present invention has a means for releasing restriction of the rotating motion of the ring gear with respect to a casing. This structure makes it possible to fix the ring gear to the casing or rotate the ring gear freely as required.